I have been corrected several times recently for putting a comma before a conjunction in a sentence (splitting phrases, not items in a list). To each their own style guide, but my understanding was that (using 'and' as an example):
- in the prehistoric era, the rule was to always put a comma before 'and', no matter the context
- in the modern era, there are two schools of style about 'and', commas and the final item in a list (which I am not concerned with here)
- in the modern era, you may put a comma before 'and' in a non-list to emphasise, indicate a pause, tweak meaning, etc. So that:
We will fight them on the beaches and the landing grounds.
has a slightly different meaning to
We will fight them on the beaches, and the landing grounds.
...but both are valid.
Is this comma in fact optional, or always to be discarded? Have I half-learned a (possibly out-moded) style rule without realizing it?